A novel formation and structure evolution of Cu-rich precipitates in FeCoNiCuAl high-entropy alloy

Abstract

Cu-rich precipitates were observed in the FeCoNiCuAl high-entropy alloy (HEA). In this paper, the microstructure of Cu-rich precipitates in the as-cast and annealed HEA was studied. A novel mechanism for the formation and microstructural evolution of Cu-rich precipitates has been discovered. In the solidification of alloy preparation, sphere-like precipitates with 9R close-packed (9R) or body-centred cubic (BCC) structures were formed. It was noteworthy that several sphere-like precipitates self-assembled into a large rod-like precipitate when they were close to each other. After the annealing treatment, the Cu-rich precipitates grew and part of them formed a core-shell structure. Cu atoms in the matrix adsorbed on the surface of the original precipitates to form the 9R structure shell, and the original precipitates underwent a phase transformation to form the twin face-center tetragonal (FCT) structure core. In addition, a large number of nano-clusters with sizes of about 3–8 nm were dispersed in the BCC matrix. The magnetic and mechanical properties of HEA were greatly affected by the annealing treatment. The σmax, σ0.2, HV, and Br of the alloy were improved 22.5%, 76.7%, 14.2% and 17.7% by annealing treatment, respectively. This work lays the foundation for further improving the mechanical and magnetic properties of HEAs by adjusting the amount, distribution, and structure of the precipitates.